The present invention relates to a method and apparatus for developing a latent image. More specifically, the invention relates to a magnetic roll for development systems.
The features of the present invention are useful in the printing arts and more particularly in electrophotographic printing. In the well-known process of electrophotographic printing, a charge retentive surface, typically known as a photoreceptor, is electrostatically charged, and then exposed to a light pattern of an original image to selectively discharge the surface in accordance therewith. The resulting pattern of charged and discharged areas on the photoreceptor form an electrostatic charge pattern, known as a latent image, conforming to the original image. The latent image is developed by contacting it with a finely divided electrostatically attractable powder known as xe2x80x9ctoner.xe2x80x9d Toner is held on the image areas by the electrostatic charge on the photoreceptor surface. Thus, a toner image is produced in conformity with a light image of the original being reproduced. The toner image may then be transferred to a substrate or support member (e.g., paper), and the image affixed thereto to form a permanent record of the image to be reproduced. Subsequent to development, excess toner left on the charge retentive surface is cleaned from the surface. The process is useful for light lens copying from an original or printing electronically generated or stored originals such as with a raster output scanner (ROS), where a charged surface may be imagewise discharged in a variety of ways.
In the process of electrophotographic printing, the step of conveying toner to the latent image on the photoreceptor is known as xe2x80x9cdevelopment.xe2x80x9d The object of effective development of a latent image on the photoreceptor is to convey toner particles to the latent image at a controlled rate so that the toner particles effectively adhere electrostatically to the charged areas on the latent image. A commonly used technique for development is the use of a two-component developer material, which comprises, in addition to the toner particles which are intended to adhere to the photoreceptor, a quantity of magnetic carrier granules or beads. The toner particles adhere triboelectrically to the relatively large carrier beads, which are typically made of steel. When the developer material is placed in a magnetic field, the carrier beads with the toner particles thereon form what is known as a magnetic brush, wherein the carrier beads form relatively long chains which resemble the fibers of a brush. This magnetic brush is typically created by means of a xe2x80x9cdeveloper roll.xe2x80x9d The developer roll is typically in the form of a cylindrical sleeve rotating around a fixed assembly of permanent magnets called a magnetic roll. The carrier beads form chains extending from the surface of the developer roll, and the toner particles are electrostatically attracted to the chains of carrier beads. When the magnetic brush is introduced into a development zone adjacent the electrostatic latent image on a photoreceptor, the electrostatic charge on the photoreceptor will cause the toner particles to be pulled off the carrier beads and onto the photoreceptor. Another known development technique involves a single-component developer, that is, a developer which consists entirely of toner. In a common type of single-component system, each toner particle has both an electrostatic charge (to enable the particles to adhere to the photoreceptor) and magnetic properties (to allow the particles to be magnetically conveyed to the photoreceptor). Instead of using magnetic carrier beads to form a magnetic brush, the magnetized toner particles are caused to adhere directly to a developer roll. In the development zone adjacent the electrostatic latent image on a photoreceptor, the electrostatic charge on the photoreceptor will cause the toner particles to be attracted from the developer roll to the photoreceptor.
As stated earlier, development is typically accomplished by the use of a magnetic brush. The magnetic brush is typically formed by a developer roll which is typically in the form of a cylindrical sleeve which rotates around a fixed assembly of permanent magnets. When utilizing magnetic brush-type development, the cylindrical sleeve is typically made of an electrically conductive, non-magnetically conductive material, for example, aluminum.
Prior art developer rolls for use with magnetic pressure development typically include a magnetic roll about which a sleeve is positioned. The magnetic roll may be held stationary and the sleeve rotates. Conversely, the sleeve may rotate with the magnetic roll permanently positioned. In configurations where the magnetic roll is stationary and the sleeve rotates, the segments are so positioned to attract the toner particles toward the developer nip between the developer roll and the photoconductive surface of the drum.
Prior art developer rolls have typically been manufactured with a core or body and magnets positioned on the periphery of the core. Typically the magnets are glued to the periphery of the core. The gluing of magnets to a core contributes to a series of problems. The gluing leads to positioning errors both radially and tangentially, reducing the quality of the roll. Further, add cost may be required to perform subsequent machining of the periphery of the roll to obtain needed accurate tolerances. Furthermore, the adhesive use to glue the magnets to the core may require special handling to conform to environmental and safety regulations. In addition, the gluing of the magnets to the core is a labor intensive hand operation which is very costly. Also, the use of glued magnet segments leads to a magnetic roll that is hard to disassemble for remanufacturing. While it may be difficult to remove the glue to separate the magnets from the core, it is further more difficult to remove the residual glue from the core and the magnets. It is further difficult to dispose of the residual glue and remove from the magnets and core.
Recently, magnetic rolls had been manufactured by positioning the magnetic strips around the periphery of a mold and molding the core with the magnetic strips prepositioned in the core of the mold. This manufacturing procedure utilizes an expensive molding. Further, the process is limited to urethane resins. The process is expensive in that the curing time for the molding operation may be extensive. Also the elevated temperatures required result in long cure times. The requirement that the process utilize urethane foam limits the flexibility of the process and the limited strength and durability of the urethane foam affect the quality and suitability of this type of magnetic roll in many applications.
The magnetic roll of the present invention is intended to alleviate at least some of the aforementioned problems.
The following disclosures may be relevant to various aspects of the present invention:
U.S. Pat. No. 5,453,471 discloses a hollow member which serves as a cylinder having an inner configuration which matches the outer configuration of a magnet roller to be manufactured. The member is mounted in a metallic mold and then the metallic mold is clamped. A molten resin containing magnetic particles is injected into the mold cavity of the hollow member through a runner.
U.S. Pat. No. 5,384,957 discloses a method of producing a magnet roll in which a magnetic property comparable to that obtained by injection molding can be obtained in spite of an extrusion process. According to a first embodiment, the yoke width of the magnetic field extrusion die is varied along an extrusion direction. According to a second embodiment, a pipe filled with resin bonded magnet material is used as a shaft.
U.S. Pat. No. 5,030,937 discloses a magnet roll for an electro-photographic device. The roll includes a magnet carrier assembly constituted by a plurality of identical cylindrical segments of injection molded plastic material. The segments are coaxially arranged and longitudinally aligned in an end-to-end relationship on a spindle like metal rod constituting the magnet roll axis of rotation. The bottom of each channel has along its length a central groove that functions as a locator for an extruded magnetic strip.
U.S. Pat. No. 5,019,796 discloses an improved bar magnet and method of construction and an improved magnetic core. An assembly of magnet is shown for use in a processing station of a printing machine. The bar magnet is formed of permanent magnet material having magnetic domains therein that are magnetized along epicyclical curve segments. The external magnetic flux density is improved over that of a conventionally magnetized magnet.
U.S. Pat. No. 4,872,418 discloses a magnet roll including a main body portion of a soft material and having a surface portion which is permanently magnetized. The roll also has a supporting portion integrally formed with the main body portion by the some soft materials a that of the main body portion for mounting the body portion to a member to which the main body is to be mounted.
U.S. Pat. No. 4,823,102 discloses a magnetic roll which is used in a processing station of a printing machine. The roll has a central portion with a plurality of spaced fins extending generally radially therefrom. A shaft extends outwardly from opposed ends of the central portion along the longitudinal axis thereof. A magnet is secured in each space between adjacent fins. A sleeve is rotatably supported on the shaft.
U.S. Pat. No. 4,804,971 discloses a cylindrical magnet for a magnetic brush development unit used in a printing machine. The magnet is of a U-shaped cross section having a cylindrical outer sleeve and a cavity through which extends the rotary axis of the sleeve. The material forming the magnet is a moldable plastic.
U.S. Pat. No. 4,608,737 discloses a magnet roll for use in a developer unit of an electrostatic copier having a magnet structure provided by elongated bars of permanent magnet material magnetized to provide radially oriented magnets. The bars are sufficiently rigid to support hubs without the need of a core. A cylindrical shell of conductive material is rotatably mounted on the magnet structure. The bars are made of conductive plastic, ceramic or rubber with a rigid steel backing.
U.S. Pat. No. 4,604,042 discloses a mold for producing an anisotropic magnet from a composition consisting essentially of magnetic powder and a binder. The mold includes a mold body, a cavity for molding the composition, yokes and first and second magnets on both sides of the yokes for preventing leakage of the magnetic field.
U.S. Pat. No. 4,557,582 discloses a magnet roll including magnet pieces adhesively secured to a supporting shaft to increase the magnetic flux density of a pole. The pieces are disposed do that they have repelling magnetic forces in the interface between the piece have the pole and the piece adjacent thereto.
U.S. Pat. No. 4,517,719 discloses a magnetic roll having a plurality of magnets integrally set fast with a retaining member to form a magnetic force generating part. The retaining member is made of a rigid synthetic resin or resin foam and a groove is provided outside of the magnetic force generating part.
In accordance with one aspect of the present invention, there is provided a method for manufacturing a magnetic roll for use in an electrophotographic printing machine of the type having an electrostatic latent image recorded on a photoconductive member. The method includes the steps of placing a shaft in a mold cavity and molding a core in the mold cavity with the shaft in the cavity. The core defines a pocket on the periphery of the core. The method further includes the step of attaching a magnet to the pocket.
In accordance with another aspect of the present invention, there is provided a magnetic roll for use in an electrophotographic printing machine of the type having an electrostatic latent image recorded on a photoconductive member in which a magnetic field attracts magnetic particles to form a magnetic brush on a sleeve surrounding a portion of the roll. The magnetic roll includes an elongated member and a core made of a moldable material. The core is molded onto the member. The core defines a pocket located on the periphery of the core. The magnetic roll further includes a magnet secured to the pocket.
In accordance with yet another aspect of the present invention, there is provided a developer unit for use in an electrophotographic printing machine of the type having an electrostatic latent image recorded on a photoconductive member. The developer unit includes a housing defining a chamber for storing a supply of toner particles therein and a magnetic roll for transporting the toner particles on a sleeve surrounding a portion of the roll from the chamber of the housing to the member. The magnetic roll includes an elongated member and a core made of a moldable material. The core is molded onto the elongated member. The core defines a pocket located on the periphery of the core. The magnetic roll further includes a magnet secured to the pocket.
In accordance with a further aspect of the present invention, there is provided an electrographic printing machine of the type having an electrostatic latent image recorded on a photoconductive member. The printing machine includes a housing defining a chamber for storing a supply of toner particles therein and a magnetic roll for transporting the toner particles on a sleeve surrounding a portion of the roll from the chamber of the housing to the member. The magnetic roll includes an elongated member and a core made of a moldable material. The core is molded onto the elongated member The core defines a pocket located on the periphery of the core. The magnetic roll further includes a magnet secured to the pocket.